example-4.js

function getElementsByClassName2(classNameStr) {

   const elements = []
   const firstChildren = this.children

   function checkChildren(child) {

      if (child.classList.contains(classNameStr)) {
         elements.push(child)
      }

      if (child.children) {
         const children = child.children
         children.forEach(child => checkChildren(child))
     }
   }

   // call the checkChildren method on the firstChildren
   firstChildren.forEach(child => {
      checkChildren(child)
   })

    return elements
}